Not parent commenter, just sharing my solution. I use Zim[0] because it already saves a folder hierarchy of markdown TXT files. The base folder is synced to Dropbox, and the markdown is readable/editable enough if you open it on a mobile device.

I'd love to use CherryTree[1] because it supports encryption and is more functional, but it stores everything in a single XML/SQLite file. Neither Zim or CherryTree[1] have mobile apps.

I really tried to use Joplin[2], which saves markdown too and has mobile apps, but the desktop app is huge. I prefer to use those resources for Keybase.

0. https://zim-wiki.org

1. https://www.giuspen.com/cherrytree

2. https://joplinapp.org